An Unprecedented Trinuclear Structure Involving Two High-Spin and

Inorg. Chem. , 2004, 43 (15), pp 4590–4594. DOI: 10.1021/ic049877f. Publication Date (Web): June 23, 2004. Copyright © 2004 American Chemical Socie...
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Inorg. Chem. 2004, 43, 4590−4594

An Unprecedented Trinuclear Structure Involving Two High-Spin and One Spin-Crossover Iron(II) Centers George Psomas,† Nicolas Bre´fuel, Franc¸ oise Dahan, and Jean-Pierre Tuchagues* Laboratoire de Chimie de Coordination, UPR CNRS 8241, 205 route de Narbonne, 31077 Toulouse Cedex 04, France Received January 30, 2004

Anaerobic reaction of ferrous thiocyanate with the deprotonated form of the pentadentate dinucleating Schiff base 1,3-bis[(2-pyridylmethyl)imino]propan-2-ol (LH) yields the novel trinuclear [Fe3L2(NCS)4(H2O)] species 1. LH results from the bis-condensation of 2-acetyl-pyridine with 1,3-diaminopropan-2-ol and includes an N4O donor set. The X-ray crystal structure of 1 [C38H40N12O3S4Fe3, triclinic, space group P-1; a ) 10.7730(10) Å, b ) 12.2048(14) Å, c ) 19.0559(19) Å, R ) 76.908(12)°, β ) 89.106(12)°, γ ) 79.637(12)°, V ) 2399.8(4) Å3] can be described either as a bent linear arrangement of ferrous centers pairwise bridged through the alkoxo oxygen atom of L- or as a triangular FeII3 core with an Fe2−SCN−Fe3 bridge as the longer side of the Fe1−Fe2−Fe3 triangle. The metric parameters characterizing the ligand environments of the three ferrous centers in 1 and its Mo¨ssbauer spectra show that this unprecedented trinuclear structure involves two high-spin (Fe2 and Fe3) and one spincrossover (Fe1) FeII centers. The donor set to the spin-crossover center (Fe1) is unprecedented: two Npyridine, two Nimine, and two Oalkoxo. Weak antiferromagnetic interactions transmitted through the end-to-end NCS bridge and/or through the O1−Fe1−O2 bridge operate between Fe2 and Fe3.

Introduction The recent impetus in the search for new spin-crossover (SCO) materials stems from the potential interest of such bistable molecular species (molecular switches) in further miniaturization of electronic devices, i.e., molecular electronics and spintronics.1 SCO materials based on discrete polynuclear ferrous complexes are scarce and most often limited to dinuclear2 or trinuclear3 species, although a * To whom correspondence should be addressed. E-mail: tuchague@ lcc-toulouse.fr. † Present address: Institute of Physical Chemistry, NCSR Demokritos 15310 Aghia Paraskevi, Attikis, Greece. (1) (a) Gu¨tlich, P.; Hauser, A.; Spiering, H. Angew. Chem. Int. Ed. Engl. 1994, 33, 2024. (b) Real, J. A.; Andres, E.; Munoz, M. C.; Julve, M.; Granier, T.; Bousseksou, A.; Varret, F. Science 1995, 268, 265. (c) Kahn, O.; Martinez, C. J. Science 1998, 279, 44. (d) Hauser, A.; Jeftic, J.; Romstedt, H.; Hinek, R.; Spiering, H. Coord. Chem. ReV. 1999, 190-192, 471. (e) Hayami, S.; Gu, Z.-Z.; Yoshiki, H.; Fujishima, A.; Sato, O. J. Am. Chem. Soc. 2000, 123, 11644. (f) Sunatsuki, Y.; Ikuta, Y.; Matsumoto, N.; Y.; Kojima, M.; Iijima, S.; Hayami, S.; Maeda, Y.; Kaizaki, S.; Dahan, F.; Tuchagues, J.-P. Angew. Chem., Int. Ed. 2003, 42, 1614. (2) See, for example: (a) Real, J. A.; Castro, I.; Bousseksou, A.; Verdaguer, M.; Burriel, R.; Castro, M.; Linares, J.; Varret, F. Inorg. Chem. 1997, 36, 455. (b) Letard, J.-F.; Real, J. A.; Moliner, N.; Gaspar, A. B.; Capes, L.; Cador, O.; Kahn, O. J. Am. Chem. Soc. 1999, 121, 10630. (c) Ksenofontov, V.; Spiering, H.; Reiman, S.; Garcia, Y.; Gaspar, A. B.; Moliner, N.; Real, J. A.; Gu¨tlich, P. Chem. Phys. Lett. 2001, 348, 381.

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tetranuclear SCO compound has recently been reported.4 The iron ligand environment of FeII SCO materials is overwhelmingly of the N6 type.5 However, very few exceptions have been reported that deserve an extensive exploration of the ability of different types of donor sets to generate SCO materials with remarkable properties: five-coordinate FeII compounds with NP2I2 and NP2INNCS donor sets exhibit unusual spin transitions,6 one six-coordinate FeII compound with a P4Cl2 donor set also exhibits SCO,7 and one sixcoordinate FeII compound with a N4O2 donor set (two Namine, two Nimine, and two Ophenoxo) exhibits a two-step spin transition with hysteresis.8 In this article, we report on an (3) Kolnaar, J. J. A.; van Dijk, G.; Kooijman, H.; Spek, A. L.; Ksenofontov, V. G.; Gu¨tlich, P.; Haasnoot, J. G.; Reedijk, J. Inorg. Chem. 1997, 36, 2433 and references therein. (4) Ruben, M.; Breuning, E.; Lehn, J.-M.; Ksenofontov, V.; Renz, F.; Gu¨tlich, P.; Vaughan, G. B. M. Chem. Eur. J. 2003, 9, 4422 and references therein. (5) See, for example: (a) Gu¨tlich, P. Struct. Bonding (Berlin) 1981, 44, 83. (b) Ko¨nig, E. Struct. Bonding (Berlin) 1991, 76, 51. (c) Magnetism: A Supramolecular Function; Kahn, O., Ed.; NATO ASI Series C; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1996; Vol. 484. (6) Kelly, W. S. J.; Ford, G. H.; Nelson, S. M. J. Chem. Soc. A 1971, 388. (7) Levason, W.; McAuliffe, C. A.; Khan, M. M.; Nelson, S. M. J. Chem. Soc., Dalton Trans. 1975, 1778.

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FeII3 Structure with Two High-Spin and One Spin-CrossoWer Centers

unprecedented trinuclear structure involving two high-spin (HS) and one SCO iron(II) centers; the ligand environment of the latter is of N4O2 type: two Npyridine, two Nimine, and two Oalkoxo. Experimental Section Materials. All chemicals, unless otherwise stated, were commercially available and were used as received. Ferrous thiocyanate was prepared in methanol with a method adapted from Erickson et al.9 Solvents were analytically pure and were deoxygenated prior to use. Synthesis. Fe3L2(SCN)4(H2O), 1. 2-Acetylpyridine (484 mg, 4 mmol) was slowly added under stirring to a methanolic solution (30 mL) of 1,3-diaminopropan-2-ol (180 mg, 2 mmol). The resulting colorless solution was stirred for 1 h, and 108 mg (2 mmol) of freshly prepared sodium methanolate were added to deprotonate the central alcoholic function of 1,3-bis[(2-pyridylmethyl)imino]propan-2-ol (LH). The reaction mixture was stirred for an additional 1 h. To the resulting light brown LNa solution was slowly added a methanolic solution (30 mL) of ferrous thiocyanate (3 mmol). The reaction mixture, which immediately turned deep blue, was stirred for an additional 1 h, was then allowed to stand for 2 days, and was finally filtered. Deep blue crystals were obtained after allowing slow evaporation of the filtrate for a few days. Yield: 400 mg, 39%. Anal. Calcd for Fe3L2(SCN)4(H2O) (C38H40N12O3S4Fe3): C, 45.25; H, 4.00; N 16.67; S, 12.71; Fe, 16.61. Found: C, 45.37; H, 3.71; N, 16.02; S, 12.55; Fe, 16.54. IR (KBr pellet, cm-1): ν(O-H), 3435 (m, br); ν(CtN)NCS, 2052(vs); ν(CdN)imine, 1647(s), 1595 (s); ν(CdS)NCS, 745 (m); π(C-H), 778 (s); δ(NCS), 480 (w). Crystallographic Data Collection and Structure Determination of 1. Single-crystal X-ray data collection was carried out at 180(2) K on a Stoe Imaging Plate Diffraction System (IPDS) using graphite-monochromated Mo KR radiation and equipped with an Oxford Cryostream Cooler Device. Final unit cell parameters were obtained by least-squares refinement of the setting angles of a set of 5000 reflections [I > 10σ(I)]. The crystal decay was monitored by measuring 200 reflections per image. No significant fluctuations of diffracted intensities were observed during the measurements. A total of 22941 reflections (8635 independent) were collected; 5257 observed reflections (Rint ) 0.0673) were used in the refinements. The structure was solved by direct methods using the SHELXS-97 program10 and refined by least-squares procedures on Fo2 with SHELXL-9711 by minimizing the function Σw(Fo2 - Fc2)2, where Fo and Fc are, respectively, the observed and calculated structure factors. The atomic scattering factors were taken from the standard compilation.12 All atoms were located on difference Fourier maps. All non-hydrogen atoms were refined anisotropically; H atoms (riding model) were refined isotropically. Weighted R factors, wR, and goodness of fit, S, are based on Fo2; conventional R factors, R, are based on Fo, with Fo set to zero for negative Fo2. Drawings of the molecule were performed with the program ZORTEP13 with 50% of probability displacement ellipsoids for non(8) Boinnard, D.; Bousseksou, A.; Dworkin, A.; Savariault, J.-M.; Varret, F.; Tuchagues, J.-P. Inorg. Chem. 1994, 33, 271. (9) Erickson, N. E.; Sutin, N. Inorg. Chem. 1966, 5, 1834. (10) Sheldrick, G. M. SHELXS-97. Program for Crystal Structure Solution; University of Go¨ttingen: Go¨ttingen, Germany, 1990. (11) Sheldrick, G. M. SHELXL-97. Program for the Refinement of Crystal Structures from Diffraction Data; University of Go¨ttingen: Go¨ttingen, Germany, 1997. (12) International Tables for Crystallography”; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1992; Vol. C, Tables 4.2.6.8 and 6.1.1.4.

Table 1. Crystallographic Data for [Fe3L2(NCS)4(H2O)], 1

formula space group a, Å b, Å c, Å V, Å3 λ, Å µ(Mo KR), cm-1 wR(obs,all)b a

C38H40N12O3S4Fe3 P1h (No. 2) 10.7730(10) 12.2048(14) 19.0559(19) 2399.8(4) 0.71073 1.075 0.1225, 0.1283

fw temp, K R, deg β, deg γ, deg Z Fcalcd, g cm-3 R(obs,all)a

1026.65 180(2) 76.908(12) 89.106(12) 79.637(12) 2 1.421 0.0584, 0.0761

R ) ∑||Fo| - |Fc||/∑|Fo|. b Rw ) [∑w(|Fo|2 - |Fc|2)2/∑w|Fo2|2]1/2.

hydrogen atoms. Crystal data collection and refinement parameters are reported in Table 1, and selected bond distances are gathered in Table 2. Physical Measurements. Microanalyses for C, H, N, and S were performed by the Microanalytical Laboratory of the Laboratoire de Chimie de Coordination at Toulouse and at the Service Central de Microanalyses du CNRS in Vernaison, France, for Fe. Infrared spectra (4000-400 cm-1) were recorded as KBr disks on a PerkinElmer Spectrum GX FT-IR spectrometer. Mo¨ssbauer measurements were recorded on a constant-acceleration conventional spectrometer with a 50 mCi source of 57Co (Rh matrix). The absorber was a powdered sample enclosed in a 20-mm-diameter cylindrical, plastic sample holder, the size of which had been determined to optimize the absorption. Variable-temperature spectra were obtained in the 80-300 K range, by using a MD 306 Oxford cryostat, the thermal scanning being monitored by an Oxford ITC4 servocontrol device ((0.1 K accuracy). A least-squares computer program14 was used to fit the Mo¨ssbauer parameters and determine their standard deviations of statistical origin (given in parentheses). Isomer shift values (δ) are relative to iron foil at 293 K. Variable-temperature (2-300 K) magnetic susceptibility data were collected on a powdered microcrystalline sample with a Quantum Design MPMS SQUID susceptometer. Data were corrected with the standard procedure for the contribution of the sample holder and for diamagnetism of the sample.

Results and Discussion Synthesis. [Fe3L2(NCS)4(H2O)], 1, results from the anaerobic reaction of ferrous thiocyanate with the deprotonated form of the pentadentate dinucleating Schiff base 1,3-bis[(2-pyridylmethyl)imino]propan-2-ol (LH, Chart 1) including a N4O donor set and resulting from the bis-condensation of 2-acetyl-pyridine with 1,3-diaminopropan-2-ol. The adjacent five-membered -Fe-N-C-C-O- metallacycles resulting from the bridging of two FeII cations by the binucleating L- ligand preclude the presence of an additional monatomic bridge, and the absence of potential triatomic bridge such as carboxylate does not allow formation of dinuclear species similar to those previously described for related pentadentate Schiff bases with N4O donor sets (1,4-bis{[2-pyridyl(1-ethyl)imino]}butane-2-ol and 1,5-bis{[2-pyridyl(1-ethyl)imino]}pentane-3-ol).15 The 3:2 Fe/LChart 1. Pentadentate Dinucleating Schiff Base 1,3-Bis[(2-pyridylmethyl)imino]propan-2-ol (LH)

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Psomas et al.

Figure 1. ORTEP view of [Fe3L2(NCS)4(H2O)], 1 with atom numbering scheme showing 50% probability ellipsoids. Table 2. Selected Bond Distances (Å) for [Fe3L2(NCS)4(H2O)], 1 bond

length (Å)

bond

length (Å)

Fe1-N1 Fe1-N3 Fe1-O1 Fe2-N5 Fe2-N9 Fe2-O1 Fe3-N7 Fe3-N11 Fe3-O2

2.054(4) 2.046(4) 2.022(3) 2.108(3) 2.106(5) 2.022(3) 2.123(5) 2.161(4) 2.044(3)

Fe1-N2 Fe1-N4 Fe1-O2 Fe2-N6 Fe2-N10 Fe2-S3 Fe3-N8 Fe3-N12 Fe3-O3

1.991(3) 1.977(4) 1.992(3) 2.215(3) 2.031(4) 3.063(2) 2.244(4) 2.074(5) 2.210(4)

stoichiometry then allows better satisfaction of the iron coordinative requirement, and the resulting trinuclear species is further stabilized through pseudo-cyclization involving an end-to-end NCS- anion. Molecular Structure of 1. The X-ray crystal structure of [Fe3L2(NCS)4(H2O)] at 180 K (Figure 1) can be described either as a bent linear arrangement of ferrous centers pairwise bridged through the alkoxo oxygen atom of L(Fe1-O1-Fe2, 129.7°; Fe1‚‚‚Fe2, 3.660 Å; Fe1-O2-Fe3, 130.4°; Fe1‚‚‚Fe3, 3.665 Å) or as a triangular FeII3 core, provided that the Fe2‚‚‚S3 interaction (3.063 Å, N9-Fe2-S3 ) 174.4°) can be regarded as a weak bond yielding an Fe2-SCN-Fe3 bridge as the longer side (5.642 Å) of the Fe1-Fe2-Fe3 triangle (Figure 1). This end-to-end thiocyanato bridging mode has already been observed, with corresponding M-S distances either in the 2.6-2.9 Å range16 or in the longer 3.11-3.18 Å range,15 the latter suggesting weak Fe‚‚‚S interactions rather than the covalent bonding suggested by the former. The intermediate 3.063 Å value observed for [Fe3L2(NCS)4(H2O)] is close to the lower limit for weak Fe‚‚‚S interactions, preventing us from qualifying this Fe2‚‚‚S3 interaction as a covalent bonding. (13) Farrugia, L. J. ORTEP32 for Windows. J. Appl. Crystallogr. 1997, 30, 565. (14) Lagarec, K. Recoil, Mo¨ssbauer Analysis Software for Windows. Available at http://www.physics.uottawa.ca/∼recoil. (15) Boudalis, A. K.; Clemente-Juan, J.-M.; Dahan, F.; Tuchagues, J.-P. Inorg. Chem. 2004, 43, 1574. (16) (a) Vicente, R.; Escuer, A.; Pen˜alba, E.; Solans, X.; Font-Bardia, M. Inorg. Chim. Acta 1997, 255, 7. (b) Go´mez-Saiz, P.; Garcı´a-Tojal, J.; Arna´iz, F. J.; Maestro, M. J.; Lezama, L.; Rojo, T. Inorg. Chem. Commun. 2003, 6, 558. (c) Maji, T. K.; Mostafa, G.; Clemente-Juan, J. M.; Ribas, J.; Lloret, F.; Okamoto, K.; Chaudhuri, N. R. Eur. J. Inorg. Chem. 2003, 1005.

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Figure 2. Representative least-squares-fitted Mo¨ssbauer spectra of [Fe3L2(NCS)4(H2O)], 1.

The three coordination polyhedra have distinct and unique features: the distorted octahedron around Fe1 involves two Npyridine, two Nimine, and two Oalkoxo, one of each from each L- ligand; the distorted pseudo-octahedron around Fe2 involves one Npyridine, one Nimine, two Nthiocyanato, one Oalkoxo, and one Sthiocyanato; the distorted octahedron around Fe3 involves one Npyridine, one Nimine, two Nthiocyanato, one Oalkoxo, and one Oaquo. Whereas the donor sets to Fe1 and Fe3 are of the N4O2 type, although different, the donor set to Fe2 is of the uncommon N4O(S) type. All Fe-donor atom distances around Fe1 are in the 1.977(4)-2.054(4) Å range (average 2.014 Å), indicating a prevailing LS state for this FeII ion at 180 K, the temperature at which the single-crystal X-ray data were collected. The larger Fe-donor atom distances around Fe2 (average for N4O donor set, 2.096 Å; average for N4O(S) donor set, 2.258 Å) and Fe3 (average 2.143 Å) are indicative of HS states for these two FeII ions at 180 K. Mo1 ssbauer Spectroscopy. The Mo¨ssbauer spectra of ground crystals of 1 (80-300 K) were unambiguously fitted to two staggered HS and one LS quadrupole split doublets (Figure 2), provided that the area ratio of the HS doublet corresponding to the Fe2 site [N4O(S) donor set, isomer shift δ ) 1.160(3) mm/s, quadrupole splitting ∆EQ ) 2.808(6) mm/s at 80 K] was set to one-third at all temperatures. Although the molecular structure evidences different donor sets for the three FeII centers, the Mo¨ssbauer spectra could not be fitted to three HS and one LS doublets. Actually, the minute difference in nature of the donor atoms, and in symmetry of their arrangement, between the N4O2 donor sets to Fe1 and Fe3 (see Molecular Structure subsection) does not allow Mo¨ssbauer spectroscopy to distinguish HS Fe1 from Fe3. At 80 K, the parameters of the LS site are δ(LS)

FeII3 Structure with Two High-Spin and One Spin-CrossoWer Centers

Figure 3. Temperature dependence of (a) the isomer shift, (b) the quadrupole splitting, and (c) the area ratio of the FeII sites of complex 1.

) 0.334(3) mm/s and ∆EQ(LS) ) 1.320(7) mm/s, and the parameters of the HS site corresponding to Fe1 and Fe3 are δ(HS) ) 1.008(4) mm/s and ∆EQ(HS) ) 2.795(6) mm/s. The mirror variations in area ratios (toward one-third) of the two latter FeII sites between values close to 33% LS and 33% HS in the 80-120 K range and values close to 20% LS and 47% HS in the 240-300 K range clearly indicate that an incomplete SCO occurred in the 120-240 K range at either Fe1 or Fe3 (Figure 3). In view of the structural information gained at 180 K, it was then obvious that Fe1 is the SCO site in this trinuclear ferrous species. It is worth noting that the donor set to Fe1 (two Npyridine, two Nimine, two Oalkoxo) is quite similar to that evidenced in the previously reported SCO compound with a N4O2 donor set8 (two Namine, two Nimine, two Ophenoxo): indeed, the increase in ligand field strength expected for the change from two Namine to two Npyridine is approximately compensated by the decrease in ligand field strength expected for the change from two Ophenoxo to two Oalkoxo. Taking into account all observed iron sites, the overall HS area ratio is included between ∼66% (below 120 K) and ∼80% (above 240 K). The ∼33% contribution for LS FeII in the range 80-120 K is remarkable, given that the LambMo¨ssbauer factor for LS FeII is usually larger than that for HS FeII {the Lamb-Mo¨ssbauer factor depends on the product (meff)1/2ΘD, i.e., it involves both the lattice [ΘD(LS) > ΘD(HS)] and the bond strength [meff(LS) > meff(HS)] effects}.17 The SCO of Fe1 is thus most probably incomplete

not only at high temperature, but also at low temperature. On the other hand, at high temperature, the SCO goes further than the ∼80% indicated by the nonweighted Mo¨ssbauer intensities. Magnetic Properties. Magnetic susceptibility measurements were carried out on ground crystals of 1 (2-300 K) in an applied field of 1 T. The product χMT of 1 decreases smoothly from 8.7 cm3 mol-1 K at 300 K to 4.0 cm3 mol-1 K at 12 K. Below 12 K, it decreases rapidly to 2.2 cm3 mol-1 K at 2 K as a result of zero-field splitting effects. As expected from the Mo¨ssbauer study, the 8.7 cm3 mol-1 K value at 300 K is below the spin-only value for three noninteracting S ) 2 metal centers (9.0 cm3 mol-1 K), but accounts for less than ∼20% of the value for S ) 0 centers: (i) the 5T2 ground state of HS FeII is split by the ligand field and spinorbit coupling, yielding levels separated by energies on the order of kT (thermally populated in the high-temperature range), which result in larger and slightly temperaturedependent magnetic moments in the high-temperature range,18 and (ii) at high temperature, the SCO goes further than the ∼80% indicated by the nonweighted Mo¨ssbauer intensities (see Mo¨ssbauer Spectroscopy subsection). With 33% of LS FeII centers below 120 K, a χMT value of 6.0 cm3 mol-1 K would be expected for two noninteracting S ) 2 metal centers. For the reasons mentioned above in i and ii, the 6.6 cm3 mol-1 K value measured at 120 K is in slight excess of this spin-only value. However, the continuous smooth decrease below 120 K, leading to 4.0 cm3 mol-1 K at 12 K, indicates operation of antiferromagnetic interactions between the FeII centers of this trinuclear species. Considering the molecular structure of 1, this observation made between 120 and 12 K, the temperature range in which Fe1 is essentially in the S ) 0 (LS) state, implies that the magnetic interactions involve almost exclusively Fe2 and Fe3 and that they are transmitted by the end-to-end NCS bridging anion and/or through the O1-Fe1-O2 bridge. Conclusions The X-ray crystal structure of [Fe3L2(NCS)4(H2O)], 1, is better described as a triangular FeII3 core with an Fe2-SCNFe3 bridge as the longer side of the Fe1-Fe2-Fe3 triangle. This novel asymmetrical trinuclear ferrous structure includes two high-spin centers, Fe2 [N4O(S) donor set] and Fe3 (N4O2 donor set), and one spin-crossover center, Fe1, with an unprecedented donor set: two Npyridine, two Nimine, and two Oalkoxo. Although Mo¨ssbauer spectroscopy does not distinguish HS Fe1 from Fe3, because of their similar N4O2 donor sets, the ligand field generated by the donor set to Fe1 (see (17) (a) Ko¨nig, E.; Ritter, G.; Kulshreshtha, S. K. Chem. ReV. 1985, 85, 219. (b) Yousif, A.; Winkler, H.; Toftlund, H.; Trautwein, A. X.; Herber, R. H. J. Phys., Condens. Matter 1989, 1, 7103. (c) Boukheddaden, K.; Varret, F. Hyperfine Interact. 1992, 72, 349. (d) Yu, Z.; Schmitt, G.; Bo¨res, N.; Spiering, H.; Gu¨tlich, P. Hyperfine Interact. 1994, 93, 1459. (e) Jung, J.; Spiering, H.; Yu, Z.; Gu¨tlich, P. Hyperfine Interact. 1995, 95, 107. (18) (a) Kennnedy, B. J.; Murray, K. S. Inorg. Chem. 1985, 24, 1552. (b) Mabad, B.; Cassoux, P.; Tuchagues, J.-P.; Hendrickson, D. N. Inorg. Chem. 1986, 25, 1420. (c) Martinez Lorente, M. A.; Dahan, F.; Petrouleas, V.; Bousseksou, A.; Tuchagues, J.-P. Inorg. Chem. 1995, 34, 5346.

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Psomas et al. above) is large enough to locate Fe1 in the SCO range, whereas the ligand field generated by the donor set to Fe3 (Npyridine, Nimine, two Nthiocyanato, Oalkoxo, and Oaquo) is lower, because of the thiocyanato ligands, and stabilizes the HS state of Fe3. The variation in LS area ratio from ∼33% in the 80-120 K range to ∼20% in the 240-300 K range clearly indicates occurrence of a smooth and incomplete SCO at Fe1 in the 120-240 K range. A likely explanation to this smooth and incomplete SCO is that the Fe1 SCO sites are isolated from each other: indeed, not only do they constitute solely one-third of the overall FeII sites, but also no significant intermolecular interactions were observed between the trinuclear species in this material. The magnetic study shows that, in addition to the Fe1HS‚‚‚Fe2 and Fe3‚‚‚Fe1HS superexchange interactions operating only at temperatures

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where the percentage of HS Fe1 is not negligible, weak antiferromagnetic interactions operate between Fe2 and Fe3 through the end-to-end NCS bridging anion and/or the O1-Fe1-O2 bridge. Acknowledgment. The European Community is acknowledged for support through a postdoctoral grant to G.P. within the framework of TMR Contract FMRX- CT980174. Supporting Information Available: Mo¨ssbauer parameters obtained from fitting the Mo¨ssbauer spectra (80-300 K) of complex 1 (Table SI-1). Magnetic data (Table SI-2) and temperature dependence of the χMT product (Figure SI-1). X-ray crystallographic file of [Fe3L2(SCN)4(H2O)], 1, in CIF format. This material is available free of charge via the Internet at http://pubs.acs.org. IC049877F